1. Field of the Invention
This invention relates to prosthesis apparatus and methods for a total hip replacement and, more particularly to a novel, modular hip stem prosthesis apparatus and method whereby the hip stem is provided as a modular system having interchangeable stem elements and mating neck elements all of which are adapted to be releasably interconnected to provide a hip stem prosthesis that is selectively configured to conform to the specific anatomical conditions encountered during the surgical procedure.
2. The Prior Art
Numerous devices have been developed as total hip replacements with the basic feature being a system for replacing the natural ball and socket of the hip joint with an artificial ball and socket arrangement. One device referred to in the art as the Sivash prosthesis (Sivash, U.S. Pat. No. 3,820,167, Artificial Hip Joint) was developed through work that began around 1956 at the Central institute for Orthopedics and Traumatology, Moscow, Russia. This work included selection of certain metals and alloys for the device with the major emphasis being on the design of a constrained socket. According to one report the Sivash prosthesis never received major clinical or market success in part due to the difficulty of the surgical procedure and the positioning of this constrained device. In any event, the Sivash prosthesis made contributions toward the femoral stem in the area of a titanium alloy for the femoral stem with a chrome cobalt for the head articulation and titanium alloy proximal sleeves for enhanced collar/calcar contact. The original Sivash femoral stem was a round, tapered peg which led to a number of noncemented failures due to rotation of the stem in the femur. The result was that a number of these prostheses were cemented after the FDA approved the use of bone cement.
Rotation in the femoral canal remained a problem in cementless arthroplasty until the development of the fluted stem which involves a plurality of longitudinal flutes being incorporated into the distal part of the stem. Bending stiffness of the stem was reduced by adding a distal coronal slot thereby insuring minimal distal-load transfer. The coronal slot also resiliently urges the longitudinal flutes outwardly into engagement with the inner walls of the femoral canal adjacent the distal end.
Fixation of the hip stem prosthesis in the proximal femoral area has been the focus of a series of sleeves designed to be releasably mounted to the proximal end of the stem. Various configurations of this sleeve have been proposed and range among (a) cylindrical, (b) externally threaded sleeves, and (c) femoral sleeves having an exterior self-tapping, conical bone screw thread for achieving immediate, secure, mechanical fixation of the stem in the proximal femur. In place of the threaded configurations, other sleeves are configured as porous coated cones to encourage tissue ingrowth to the prosthesis. The porous coating is achieved by sintering a layer of titanium beads to the substrate. These cones are provided with or without a series of steps or ridges formed transversely around the cone.
The sleeve/stem combination was designed to provide the surgeon with a plurality of sleeve sizes and configurations received in mating relationship with a stem selected from a plurality of stem configurations. Conventionally, the mating surface is a taper having about a three degree tapered section so that when joined together the sleeve will not rotate relative to the stem.
Examples of such sleeve and stem combinations are found in U.S. Pat. Nos. 4,790,852 and 4,846,839. In each of these patents and in all other known hip stem prostheses, the sleeve is interchangeably mounted to the proximal end of the stem. The remainder of the proximal end of the stem is formed into an integral neck portion which incorporates various configurations to adapt the particular stem for a specific implant application. Accordingly, while the sleeve is interchangeable, each stem is restricted to a particular neck configuration. Further, greater diversity is encountered in the different neck configurations than between the distal portion of the particular stem and the sleeve.
In view of the foregoing it would be a significant advancement in the art to provide a hip stem and a mating neck assembly removably mounted to the stem, the stem including an integral, graduated collar system for mounting the stem in the femoral cavity. It would also be advantageous to provide a modular hip stem whereby various lengths and diameters of distal stem can be secured to the collar system to provide for a more uniform fit of the hip stem prosthesis in the femoral cavity. Another advancement would be to provide a mating neck assembly that includes a plurality of configurations and is removably mated to the stem. An even further advancement would be to provide for the replacement of the neck portion of the hip stem without necessarily also removing and replacing the distal stem portion of the hip stem. Such a novel apparatus and method is disclosed and claimed herein.